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I'm considering using 1x3 strapping before applying my new drywall ceiling (after doing some more work on them the other night and realizing it may be the best option). The ceiling joists are exposed and the walls at the top have a gap from about 1/2-3/4" from using a utility knife during tear down. I'm not overly concerned with gap in excess of 1/2" because taping and mudding will hide it, plus I'm adding crown with backing at the end.

What I want to do before installing the strapping is to ensure the joists are in the same plane with one another. I don't have a laser level, but I do have a 4' bubble level and some string. My thinking was I could run some string at various intervals across the joists, having it flat against the first and last joists and checking that it's tight against all the other joists. I could then confirm by measuring from the joists that cause the gaps (meaning they are the lowest) to the floor.

Any suggests on another way (without laser level) to achieve these?

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Your plan for using builder's string is good, but don't set the string tight. That opens you up to cumulative contact error. You'll end up with a soup bowl for a ceiling.

Instead, put a spacer of a convenient thickness under the string at the outside. I've often used a scrap of 1x or 2x lumber (3/4" and 1-1/2", respectively). Then you'd measure the gap at intervals, either by setting a series of lines across the room, or by swinging a single line from a corner across to various points along the opposing wall.

If you find high or low spots, here are a couple remedies...

  • For low spots, sister blocks or strips to the joist as needed, or use shims under your strapping. You can still screw your strapping to the joist, so these don't need to be thick enough for screws. They just act as a spacer.

  • For high spots, chalk a line from end to end on one side of the joist, anchoring at the bottom corner. Use a circular saw or power planer to remove the hump (sag).

Note that level isn't really what's important here. No one can see a mild slope in a ceiling. Flat is your priority.

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  • To your last comment, isn't finding the ceiling low point and adding straps to be level with that point across the room, ultimately going to result in a flat ceiling? I'm sure "level" and "flat" are used interchangeably, albeit incorrectly? I planned to identify this low spot and have all straps be level to that height, creating a flat surface before drywalling. For spots that need it, using shims between the joist and strap to lower the strapping to the low spot. – pstatix Dec 1 '20 at 15:02
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    @pstatix you can find the lowest spot on the ceiling then shim everything down to that spot. If however, you have 1 spot that's, say, 1" lower, that means that you have to move everything down to that level. If the rest of the ceiling is with 5/8", it may be better to trim the joist by 3/8" to bring it up to that level, then bring other spots down. You'll still end up flat, but higher and fewer shims. – FreeMan Dec 1 '20 at 15:33
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    @pstatix A diagonal line is flat; it is not level. A flat surface is something with no bumps. A level surface is something that you can set a ball on without having the ball roll away. – MonkeyZeus Dec 1 '20 at 16:47
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    @pstatix Yes, that would be the end result. Isherwood is saying that achieving a level ceiling could be more trouble than it's worth if a flat surface can deceive the eyes. Your current joists might already produce a flat surface as they stand but chasing your dream of a level ceiling could result in a non-flat surface which the eyes will quickly see. – MonkeyZeus Dec 1 '20 at 16:56
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    @MonkeyZeus It seems that Isherwoods method actually determines if they are on plane with one another because the string is never the relative measure. He suggests using a common thickness (scrap wood) and measuring the distance between the string and the joists relative to that. The string, which runs perpendicular to the joists, follows the angle of the joists. Thus, if they were out of level (diagonal) but on plane, they would all measure the same distance. The method you suggest makes them both level and flat, which doesn't require much more effort IMHO. – pstatix Dec 1 '20 at 17:57
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You're assuming that your floor is level; this will lead to issues.

What you can do is mark several level spots on the wall which runs parallel with your joists. Since you have a 4' level I recommend putting it on the wall and marking a dot every 2 feet or less so that when you go to mark your next dot you can line up the level on 2 previous dots instead of having to meticulously keep track of the bubble.

Attach string at each spot you marked.

Now run the string to the other wall (the string should be running perpendicular to the joists), use a string line level, and attach the string to the other wall.

This will let you see and measure whether your joists are all level in relation to your string.

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The approach by @isherwood is correct. However, a very simple tool I've used that may make it simpler to fine-tune the plane is an 8 foot aluminum straight edge. They're inexpensive, about $25. The one I use is anodized aluminum (light), is easily broken down into four foot sections and has virtually no flex, especially if used on edge.
By sliding it across the surface of the joists you can quickly see where high spots and low points occur. The best approach IMO is to plane down the highest spots first and then build up low areas with blocking or shims as needed.
Edit: Since this is a ceiling using a straight edge to identify irregularities is easier done with two people. One working the straight edge and the other marking the high or low points on the joists.
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